1. Field of the Invention
This invention relates to an improvement of a steel cord for use as reinforcement of tires, belts, etc.
2. Prior Art
Generally, this kind of steel cord is composed by intertwisting a plurality of material wires into a strand, for example, as shown in FIG. 1A and FIG. 1B, a steel cord 1 is composed by intertwisting four or five of equal material wire 2. Then, a plurality of steel cords thus made are covered with a rubber compound as they are arranged in parallel state, for use as reinforcement of rubber of tires, belts, etc. Indispensable requirements when such steel cords are used as reinforcement are (1) good chemical adhesion of steel cords to a rubber compound and (2) good infiltration of the rubber compound into the central part of the steel cord, in other words, in order to enable the steel cord to perform its duties as reinforcement of a rubber structure, it is required that steel cords and rubber compound are formed into a perfectly integrated substance.
In the case where steel cords are used for tires, for example, poor adhesion of steel cords to the rubber compound and unsatisfactory penetration of the rubber compound into the central part of the steel cord cause separation of steel cords from rubber compound or the so-called "separation phenomenon" during the running of a car, with a resultant reduction of the function of tires. Moreover, material wires of steel cords get rusty due to moisture contained in the rubber compound, with the result of deterioration of cord strength to a large extent and earlier occurrence of the "separation phenomenon" mentioned above.
In order to eliminate the above disadvantages of the conventional steel cord, a steel cord with rubber compound infiltrating into its central part and covering the whole circumference of component material wires has been taken into consideration. This steel cord is a steel cord of soft twist 1 with a space C left between material wires 2 as shown in FIG. 2A and FIG. 2B.
In the case of the above-mentioned steel cord of soft twist, however, in order to have the rubber compound adhere to the whole circumference of each material wire and infiltrate into the central part of the steel cord in sufficient quantity, the space C between material wires must be large enough to permit the rubber compound to infiltrate through it or 0.02 mm at the least. However, a comparatively large space between material wires can cause unstable twist construction in the manufacturing of steel cords, for example, material wires 2 are one-sided and twist becomes uneven in lengthwise direction of a steel cord. In this case, even if the rubber compound penetrates into the central part of a steel cord, elongation under a very low load increases, with a bad influence upon the tire manufacturing operation. Moreover, when buckling took place at the steel cord, concentration of stress is caused with the result that plural material wires constituting a steel cord do not display their strength as an integrated substance and are affected by stress break much earlier.